1. Field of the Invention
The present invention relates to a method for improving a semiconductor substrate having a SiGe film wherein the quality of the SiGe film is improved in regard to the surface condition, to the crystallinity and to relaxation of lattice distortion in the semiconductor substrate wherein the SiGe film is formed on a Si or SOI substrate, further, it relates to a semiconductor device manufactured by using a semiconductor substrate of which the quality has been improved.
2. Description of Related Art
A method is known wherein a distorted SiGe film having a lattice constant differing from that of Si is formed on a Si substrate in a virtual lattice form and the distortion caused within this SiGe film by the mismatch between the lattice constants of Si and SiGe is relieved through the introduction of misfit dislocations and, after that, an upper layer Si film is formed on the SiGe film as a cap layer in order to improve the mobility of electrons and holes that pass through channel regions.
This upper layer Si film is stretched by the SiGe film having a lattice constant that is greater than the Si film and, thereby, distortion is caused within the upper layer Si film and thus, the band structure is altered so as to improve carrier mobility.
A method for relaxing the distortion in a SiGe film is known wherein the thickness of the SiGe film is increased and thereby, the elastic strain energy of the SiGe film is increased so as to relax the lattice. It has been reported, for example, that by gradually increasing the Ge concentration in a SiGe film in order to form a SiGe film having a concentration gradient of approximately 1 μm, relaxation of distortion in the SiGe film can be achieved in the paper in Appl. Phys. Lett. 59 (13), 1611 (1991) by Y. J. Mii, et al.
In addition, a method is known for relaxing the distortion in a thin SiGe film formed on a Si substrate wherein an ion implantation process, such as of hydrogen, is carried out on the thin SiGe film and, after that, an annealing process is carried out at a high temperature so that layered defects that have been created in defect layers within the Si substrate cause slippage and, therefore, misfit dislocations are generated at the interface of SiGe/Si. It has been reported, for example, that relaxation of distortion can be achieved by means of hydrogen ion implantation in the paper in Appl. Phys. Lett. 76 (24), 3552 (2000) by H. Trinkaus, et al.
According to a method for achieving lattice relaxation by forming a thick SiGe film so as to increase the elastic strain energy of the SiGe film, however, the SiGe film exceeds the critical film thickness needed to gain perfect crystallinity of the SiGe film and, therefore, a very great number of defects occur in the SiGe film.
In addition, in the case of a thick film the film grows while distortion is relaxed by itself and, therefore, roughness, referred to as cross hatching, occurs at intervals of several tens of μm on the surface of the SiGe film such that the film cannot be used as it is as a semiconductor substrate and, therefore, a flattening process, such as a CMP process, becomes essential and another SiGe film must be grown on the surface of the SiGe film on the substrate on which the flattening process has been carried out.
On the other hand, according to a method for relaxing lattice distortion in a thin SiGe film by carrying out a hydrogen ion implantation process and a high temperature annealing process so as to generate misfit dislocations at the interface of SiGe/Si, the crystallinity is poor even though the distortion is sufficiently relaxed and the distortion can be relaxed without fail. Also, the surface condition of the SiGe film is worsened. Therefore, this method is not necessarily an effective method for improving quality.
That is to say, there is a problem that it is difficult to gain a sufficient distortion relaxation ratio while maintaining a good surface condition and a good crystallinity because the surface condition, the crystallinity and the distortion relaxation ratio greatly change depending on the film conditions of the SiGe film on which processes are carried out, even though the hydrogen ion implantation conditions, such as ion implantation energy and amount of implanted ions (dose), are kept constant.
Here, the distortion relaxation ratio is found according to the following equation [1].
                    R        =                                                            a                //                            ⁡                              (                x                )                                      -                                          a                //                            ⁡                              (                Si                )                                                                                        a                //                            ⁡                              (                SiGe                )                                      -                                          a                //                            ⁡                              (                Si                )                                                                        [        1        ]            
R indicates the relaxation ratio of lattice distortion in the SiGe film, a//(Si) and a//(SiGe) are the (001) lattice constants of Si and of SiGe, respectively, in the condition of complete lattice relaxation and a//(x) is the (001) lattice constant of the actually measured SiGe layer. In the case of a//(x)=a//(SiGe), the lattice is in the condition of complete relaxation where the relaxation ratio is 100%. The case of a//(x)=a//(Si) indicates the condition of complete lattice match with the Si substrate where the relaxation ratio is 0%.